Epilepsy is a devastating neurologic condition that affects about 2 million Americans and is often resistant to medical treatment. Animal studies have demonstrated that epilepsy is associated with compensatory changes in the brain, which includes the aberrant rewiring of neuronal circuits. In particular, epilepsy is associated with altered connectivity in the hippocampus, a region of the brain that is frequently the focal point for the initiation of seizures. One particular hippocampal circuit rearrangement associated with epilepsy involves the growth (?sprouting?) of hippocampal granule cell axons (the mossy fibers) in a retrograde direction. These sprouted fibers could directly cause hippocampal hyperexcitability by forming recurrent excitatory circuits, or alternatively increase the activity of inhibitory mechanisms and prevent seizures. Using a combination of novel approaches, our recent work determined that these fibers directly drive retrograde excitation and hyperexcitable circuit function. Interestingly, a large proportion of these retrograde projections derive from newly generated, adult-born granule cell neurons, which are produced in large numbers after seizures and undergo aberrant maturation and circuit integration. This suggests that these adult-born neurons might contribute substantially to seizure initiation and propagation, if they alter the balance of excitation and inhibition in the hippocampus. At the same time, sprouted mossy fibers have long been known to produce multiple peptide neurotransmitters, which include endogenous opioid peptides. Although the receptors for these peptides are known to potently control neuronal excitability throughout the brain, the functional importance of endogenous peptides in the control of hyperexcitability in epilepsy has not been explored. Notably, even basic questions regarding the conditions under which these peptides are released, the functions of specific receptors in different cell types, and whether these peptides modulate seizure frequency or severity in epilepsy are not known. Thus, their role during epileptogenesis remains a long-standing unanswered question in the field, and represents a therapeutic opportunity. With this proposal, we will answer fundamental questions regarding the roles of these peptides in the control of hippocampal function in epilepsy, and how potential alterations in opioid peptide signaling mechanisms due to enhanced neurogenesis might overwhelm endogenous control mechanisms that prevent seizures. We have combined various lines of genetically modified mice, which allow us to specifically label and optogenetically control different subsets of hippocampal granule cells in live tissue. We will induce experimental epilepsy using the well-established pilocarpine model of epilepsy, and use electrophysiologic recording techniques to study the functional roles of these peptides in the hippocampal circuit. Furthermore, we will use additional genetic manipulations to modify the electrical activity of peptide-releasing cells in epileptic mice in vivo, to determine how this affects seizures. Our work will provide insights into the function of opioid signaling in epilepsy, and allow us to determine whether sprouting from various cohorts of granule cells differentially modulates hippocampal excitability. This will answer long-standing questions regarding the pathogenesis of acquired epilepsy, by directly defining the functional role of the sprouted mossy fiber pathway and its various peptide signaling mechanisms. An understanding of the mechanisms through which opioids control hippocampal excitability could lead to a novel therapeutic approach to potentially prevent seizures after neuronal injury.